Many vehicles heretofore have employed spring-applied, hydraulically-released (SAHR) parking brake systems. In these systems, a brake such as shoe assembly or a disk assembly is engaged under the force of a spring which permanently biases the brake towards its applied or engaged position. The brake is released by hydraulic pressure generated by a pump driven directly or indirectly by the vehicle's engine.
SAHR parking brake systems have included brake override assemblies to permit manual brake override for vehicle towing or assisted transport when the engine is not running, the pump fails, or when the vehicle otherwise is incapable of transmitting hydraulic pressure to the brakes. One such brake override assembly is disclosed in U.S. Pat. No. 4,195,716, wherein a brake override assembly includes a manual cut-off valve and a manual hydraulic pump. The cut-off valve is located in a conduit leading from the system's brake control mechanism to the brake release actuators. The pump is disposed in an auxiliary conduit fluidically coupled to the brake release actuators. To release the brakes for towing or assisted transport, the cut-off valve is closed, and the pump is actuated to pressurize the brake release actuators and, hence, to release the brakes. The brakes remain disengaged unless and until the override valve is manually reset to its normal operating position in which fluid may flow from the brake release actuators to the vehicle's reservoir or tank.
U.S. Pat. No. 5,984,425 discloses a SAHR parking and emergency brake system that can be manually overridden to permit towing or assisted vehicle transport when pump pressure is unavailable for the brakes, and reset either automatically when pump pressure becomes available or manually. Override is made possible by a manual override assembly that includes a manually-actuated hand pump and an override valve. The override valve is manually actuated to isolate the brake release actuators and the outlet of the hand pump from the system's reservoir and is reset to its open position in response to the presence of hydraulic pressure at a designated location of the braking system. Automatic reset of the override valve preferably is effected by the supply of pressurized fluid to a previously unpressurized port of the override valve. The hand pump, override valve, and any other brake control valve(s) of the braking system may be incorporated into a single control module.